In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a variety of U.S. patents assigned to the assignee of this patent application, including U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,641,135; 5,642,122, and 5,650,788. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth orbit (LEO) satellites that implement an Earth-fixed cellular beam approach to transmitting data from one location on the Earth's surface to another location. More specifically, each LEO satellite has a communication "footprint" that covers a portion of the Earth's surface as a satellite passes over the Earth. The communication footprint defines the area of the Earth within which ground terminals can communicate with the satellite. Located within each footprint are a large number of cells. During the period of time a cell remains within the borders of a satellite footprint, ground terminals located in the cell transmit data to and receive data from the "servicing" satellite. When a satellite reaches the end of its servicing arc, another satellite in orbit is positioned to "service" the Earth-fixed cells previously covered by the satellite reaching the end of its servicing arc. During servicing, the antennas of ground terminals located in the cells continuously point toward the servicing satellite as it moves in orbit and antennas on the satellite point toward the cells during the time period within which the ground terminals in the cells are allowed to transmit data. Other LEO satellite communication systems employ a satellite-fixed beam approach to transmitting data from one location on the Earth's surface to another location.
Regardless of the nature of the LEO satellite communication system, Earth-fixed cell or satellite-fixed beam, data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within a cell to the satellite serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites to the satellite serving the cell within which the ground terminal of the designated receiver is located. The latter satellite transmits the data to the receiver ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a satellite data communication network wherein each ground terminal and satellite forms a node of the network.
In order for a LEO satellite data communication system to be competitive, it must have a wide bandwidth and be of relatively low cost. Low cost requires that the satellites be light in weight and relatively inexpensive to manufacture. One way of keeping satellite weight and cost low is to minimize the complexity of electronic signal processing hardware, and keep data transmission and reception power requirements low. Unfortunately, low data transmission and reception power conflicts with the need for a highly reliable data communication system because it is relatively easy to lose data contained in low-power signals. One way of improving the reliability of low-power data communication signals that is well known in the satellite communication field is to forward error correction (FEC) code the data to be transmitted. See U.S. Pat. Nos. 5,117,427; 5,446,747; and 5,473,601 for examples of FEC coding of digital data signals.
Typically, data transmissions are broken into digital data "packets" each of which include a header and a payload. The header data packets contain address and control information designed to direct the data packets through the satellite constellation to a desired ground terminal. The payload contains the information being transmitted, which is intended for the satellite or the ground terminal or both. A prior approach of transmitting the data involved either completely or partially FEC decoding and then re-encoding the header and payload data at the satellites. Since at least some payload decoding and re-encoding are required on the satellites with this approach, satellite power and complexity requirements are greater than they would be if no payload decoding and re-encoding occur on the satellites. Furthermore, once the satellites are in orbit, changes to the coding scheme are not possible.
The present invention is directed to a LEO satellite data communication system that uses FEC coding in a novel way to minimize power requirements, minimize complexity of the satellites, and maximize reliability, in part by requiring minimal decoding and re-encoding of the payload data on the satellites.